This invention relates to a method for obtaining in large volume a gas stream that is 90%-99% and higher by volume in one component of a gaseous mixture. This invention especially relates to an adsorption process for providing an enriched gas stream by means of a pressure swing adsorption system using carbon molecular sieves.
More particularly, this invention relates to a method for providing an inexpensive and high volume source of gases such as nitrogen, hydrogen or methane, requiring less energy to operate than either cryogenic or other pressure swing adsorption systems, and yet supplying gases of comparable quality.
The term gaseous mixture, as used herein, refers to air and other gas mixtures primarily comprised of at least two components of different molecular size. The term enriched gas refers to a gas comprised of that component of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through a two column adsorption zone (connected in series). The term lean gas refers to a gas passed through only one column of a two column adsorption zone having a fraction of undesirable components less than that of the starting gaseous mixture but more than that of the desired product gas.
A gaseous mixture may be fractionated, or separated, using pressure swing adsorption by passing the mixture at an elevated pressure, hereinafter referred to as the adsorption pressure, through a column of adsorbent which is selective in its capacity to adsorb one or more of the components of the mixture. This selectivity is governed by the pore size distribution in the adsorbent and the pore volume of the proper pore size for adsorption of a particular gas component. Thus, gas molecules with a kinetic diameter less than or equal to the pore size are retained, or adsorbed, on the adsorbent while gas molecules of larger diameters pass through the column. The adsorbent, in effect, sieves the gas according to the component's molecular size. The gaseous mixture may also be fractionated because of different rates of diffusion of its components into the pore system of the adsorbent.
As the gas travels through the adsorbent column, the pores are filled with gas molecules. One can envision an adsorption front, moving through the column, akin to the liquid adsorption front moving through a solid adsorbent in a column chromatography system. After some time the gas exiting the column is essentially the same in composition as the gas that entered the adsorbent. This is known as the "breakthrough" point. At some time before this breakthrough point, the column must be regenerated.
After treatment of the mixture to adsorb selected components therefrom, the flow of the gaseous mixture through the column is interrupted and the adsorbent is regenerated for reuse by purging it of the adsorbed components either by vacuum or by passing through the column, generally in the opposite direction of flow taken by the gaseous mixture, a purge gas stream which may comprise a portion of the purified product at a low pressure.
Pressure swing adsorption usually includes at least two columns of adsorbent so that while one column is being regenerated, the other is in the adsorption phase producing product gas. Thus, by cycling between the columns product gas is obtained constantly. The term adsorption zone, as used herein, refers to a serial arrangement of two adsorption columns, i.e., during adsorption, gas enters the inlet of the first column in the zone and exits the zone via the outlet of the second column comprising the zone. When using two such zones, by cycling between these zones, product gas is obtained constantly.
The recovery of oxygen enriched air utilizing an adsorption process employing siliceous or carbon containing adsorption agents and involving the use of temperature or pressure changes during adsorption and desorption is well known. See for example, Nandi and Walker, Separation Science 11 441 (1976), "Separation of Oxygen and Nitrogen Using 5 A Zeolite and Carbon Molecular Sieves." Certain silicates, as for example zeolites, are effective for preferably adsorbing nitrogen from its mixtures with oxygen so that by conducting air through a zeolite filled column, the first issuing gas is effectively enriched in oxygen content. The regeneration of zeolites however requires considerable expense in terms of energy and apparatus. For example Wilson in U.S. Pat. No. 3,164,454 describes the separation of oxygen from air using zeolites.
A well known process is the use of carbon molecular sieves for the production of enriched nitrogen from air. See for example, Vesterdal, U.S. Pat. No. 2,556,859 and Munzner et al., U.S. Pat. No. 3,960,522. These sieves possess a pore structure with a size comparable to the kinetic diameter of oxygem. When used in a pressure swing adsorption systen, these sieves selectively adsorb oxygen from a gas mixture, allowing other components to pass.
A four column pressure swing adsorption unit has been successfully employed in the separation of hydrogen gas from its mixture with carbon dioxide, water and light aliphatic hydrocarbons. See for example, Wagner in U.S. Pat. No. 3,430,418.
Also well known is the fractionation of other binary gas mixtures by pressure swing adsorption. For example, carbon monoxide from its mixture with hydrogen using zeolite 13X and carbon dioxide from its mixture with fuel gas mixtures using charcoal, alumina or silica. See, Simonet, U.S. Pat. No. 3,884,661.
Binary gas mixtures of argon and oxygen or helium and methane have been separated on an adsorbent of partially oxidized carbon in a pressure swing adsorption process. See, German Auslegungsschrift No. 2,045,200.
Typical problems in the present carbon molecular sieve technology include; low yield of product gas, large amounts of molecular sieve required and energy inefficient regeneration methods.